Lately there have been some articles and youtube videos about firing guns underwater. But the concept of firing a standard firearm underwater has little practical value: no useful range, unreliable weapon functioning, greatly reduced stopping power. Since the invention of SCUBA and Draeger diving equipment, underwater military and espionage missions became viable, and allegedly commonplace. And other than the sporting speargun, there was no class of weapons that could be employed underwater.
There was only so much that trained seals, dolphins, and narwhals can do offensively. So, naturally, underwater weapons needed to be invented.
(click the title for more)
The Soviet Union and NATO both developed underwater guns. The Russian examples are the APS underwater assault rifle, and the SPP-1 underwater pistol. Both fire ammunition that looks like a standard rifle shell with a long blunt-nosed dart. While they use different ammunition (5.66mm x 120mm in a 26-round magazine for the APS, and 4.5mm x 115mm in a fixed 4-barrel derringer configuration for the SPP-1/SPP-1M pistols), both employ the principle of supercavitation to achieve adequate range & velocity.
The blunt nose of the dart pushes the water aside so far that the rest of the dart is inside a bubble with no water touching it, thus no friction. More velocity and range than a speargun, which does not supercavitate. While clever, there is a very real drawback to this technique: the range of the projectile is highly dependent upon the depth at which the projectile is fired. More depth = more water pressure = smaller cavity = more friction on the projectile = less range. The pistols, for example, claim a 17-meter range at 5 meters depth, but only 11 meter range at 20 meters and 6 meter range at 40 meters (the rifle claims 30-, 20-, and 11-meter ranges at those depths). Still, at typical scuba depths these would be effective.
NATO had their own solutions to the problem. The first concept was the American MK1 Underwater Defense Gun, which fired 4.25″ heavy tungsten darts in a 6-round pepperbox configuration, spin-stabilized by 4 fins at the rear of the dart. It was allegedly bulky and unpopular with the troops tasked to use it. The claimed range of 30 feet (approx 9-10 meters) is shorter than the Russian equivalent.
HK then developed the P11 to see if they could build a better mousetrap. Details are sketchy, but is supposed to fire 4″ finned darts from a 5-round pepperbox design that must be reloaded at the factory. Claimed range is 10-15 meters.
China is supposed to have pirated the Russian designs, but developed their own larger-caliber (5.8mm) 3-barreled pistol with the same general pattern as the SPP-1.
Given the technical knowledge of supercavitation that the Russians have developed (for example, the Shkval supercavitating torpedo), it really isn’t surprising that they chose that path for their guns.